SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurement (Details)	9 Months Ended
	Sep. 30, 2018 methodology
Accounting Policies [Abstract]
Number of valuation methodologies used to determine fair value of investments	2
Maximum
Fair Value Measurements
Weighting percentage of methodology used to determine fair value of investments (up to 100%)	100.00%
Illiquidity Discount | Minimum
Fair Value Measurements
Private equity, measurement input	0.05